This invention relates to trans-2-fluorophenyl-5-(trans-4'-alkylcyclohexyl)-1,3-dioxane derivatives, and more particularly to novel liquid crystal compositions including trans-2-fluorophenyl-5-(trans-4'-alkylcyclohexyl)-1,3-dioxane derivatives suitable for use in electro-optical displays.
Liquid crystal display devices utilize electro-optical effects possessed by liquid crystals. The liquid crystal materials used in these devices have a nematic phase, a cholesteric phase and a smectic phase. The most widely used display mode uses liquid crystal materials in the nematic phase and includes the dynamic scattering type (DSM), guest-host type (G--H), twisted nematic type (TN), super-twisted nematic type (STN), super-twisted birefringence (SBE) modes and the like. The driving systems used for these liquid crystal display devices include the static driving system, time-sharing driving system (dynamic driving system), active matrix driving system, two frequency driving system and the like.
Liquid crystal display devices have several advantages, particularly when compared with conventional light emission type displays including LED devices, EL devices and CRT devices. The devices are small in size and can be made thin, the devices can be driven at low voltage with low power consumption and the devices have good compatibility with LSI and simple driving circuits. Liquid crystal display devices operate with incident light, and their displays are clearly visible even under direct sunlight. In addition, the liquid crystal material is a light receiving element so that when a liquid crystal display is viewed over a long time, eye strain does not occur. In view of these benefits, liquid crystal display technology has been applied to watches, cameras, electronic counters, audio equipment, automobile dashboard indicators, electronic games, telephone equipment, measuring devices, desk-top electronic calculators, and the like. More particularly, liquid crystal display devices have also been utilized recently in other display devices which require high resolution and many pixels.
The predominant liquid crystal display device is a TN type utilizing a time-sharing driving system. However, the maximum number of scanning lines is about 200 and attempts to increase this number have not been completely successful. In order to increase the number of scanning lines, STN mode liquid crystal display devices, SBE mode liquid crystal display devices and TN mode liquid crystal display devices driven by active matrix driving systems have been used. The STN mode is currently utilized in liquid crystal display devices and personal computers and word processors, while TN mode devices driven by active matrix driving systems are predominantly utilized in color televisions. Thus, liquid crystal display devices continue to attract attention as potentially replacing cathode ray tubes. As a result, liquid crystal display devices have been applied in various areas and it is likely that their use will be broadened further.
For practical use, liquid crystal compositions must possess the following characteristics:
1 The liquid crystal materials must be colorless and thermally, optically, electrically and chemically stable;
2. Have a wide nematic temperature range (MR), particularly over the range of normal room temperature;
3. Have a low threshold voltage (V.sub.th) of the voltage-luminance characteristic (V-I.sub.0 characteristic);
4. A small temperature dependence of threshold voltage (V.sub.th);
5. A steep rise in voltage-light transmittance (.gamma.);
6. A small V-I.sub.0 characteristic dependence on the view angle property (.alpha.); and
7 Have a rapid electro-optical response speed.
Many liquid crystal materials possess the first of the above-desired properties, however, no one compound satisfies all of the remaining characteristics. Thus, liquid crystal compositions are formed of several different liquid crystal compounds or liquid crystal compositions are obtained by mixing liquid crystal compounds with pseudo liquid crystal compounds to obtain the desired properties. Pseudo liquid crystal compounds are compounds resembling liquid crystal compounds in their molecular formulas, but fail to manifest liquid crystal phases.
It is convenient to mix liquid crystal compounds of the same type. However, liquid crystal compositions having characteristics 1-7 cannot be attained unless liquid crystal compounds of different types are mixed. The liquid crystal compositions used in TN or STN liquid crystal display devices are of the Np type. Since liquid crystal compositions formed solely of compounds of the Np type have inferior qualities, liquid crystal compositions having more desirable characteristics are obtained by mixing compounds of the Np type with compounds of the Nn type. Generally, these liquid crystal compositions include up to twenty compounds, preferably seven to eight compounds.
In general, liquid crystal compositions must have a nematic liquid crystal temperature range (MR) between at least -20.degree. and +60.degree. C. However, if a liquid crystal display device is used outdoors, for example, on an automobile dashboard, the liquid crystal composition used in the liquid crystal display device must have a wider nematic liquid crystal temperature range, preferably between about -40.degree. and +80.degree. C.
In general, liquid crystal compositions are eutectic mixtures in order to lower the lowermost limit of the nematic temperature range as low as possible. The eutectic mixture composition can be obtained from the following formulae: ##EQU1## EQU .SIGMA.X.sub.K =1
where
T.sub.L is the lowermost limit of the nematic temperature range; PA1 X.sub.K is the mole fraction of component K at T.sub.L ; PA1 .DELTA.H.sub.K is the molar heat of fusion at T.sub.K ; and PA1 R is the gas constant. PA1 .eta. is the viscosity; PA1 V is the applied voltage; and PA1 K is represented by the following formula: EQU K=K.sub.11 +(K.sub.33 -2K.sub.22)/4
The values provided by the above formulae are relatively close to the actual values if the components are of the same type. However, values provided by the above formulae are inapplicable when the components of the liquid crystal composition are of different type. Thus, personal knowledge is heavily relied upon in formulating liquid crystal compositions.
The upper limit (T.sub.u) of the nematic temperature range (MR) can be determined from the following formula: EQU T.sub.U =.SIGMA.X.sub.K T.sub.N-I,K
where T.sub.N-I,K is the nematic phase-isotropic phase transition temperature (N-I Point) of component K. Generally, the value of I.sub.U obtained using the above formula is higher than the actual value.
The components of a liquid crystal composition are classified by their phase transition point into three different types: (1) low-temperature liquid crystal compounds, (2) intermediate-temperature liquid crystal compounds, and (3) high-temperature liquid crystal compounds. Generally, to obtain a liquid crystal composition having the required characteristics described above, liquid crystal compounds of each of the three types are combined. Low-temperature liquid crystal compounds have crystal-nematic phase transition points (C-N points) or smectic-nematic phase transition points (S-N points) below about 50.degree. C. and N-I points between about 50.degree. and 70.degree. C. The low-temperature liquid crystal compounds are principal components of liquid crystal compositions and determine T.sub.L. Examples of low-temperature liquid crystal compounds include compounds having the following formulae: ##STR2## wherein R and R' are linear alkyl or alkoxy groups.
Intermediate-temperature liquid crystal compounds have C-N points between about 50.degree. and 100.degree. C. and N-I points between about 100.degree. and 200.degree. C. and may be principal components of liquid crystal compositions. Examples of intermediate-temperature liquid crystal compounds include compounds having the following formulae: ##STR3## wherein R and R' are linear alkyl or alkoxy groups and X and Y are H or F.
High temperature liquid crystal compounds have C-N points between about 80.degree. and 150.degree. C. and N-I points above about 200.degree. C. and are highly effective in raising the T.sub.U values of liquid crystal compositions. However, high-temperature liquid crystal compounds are disadvantageous since they have high molecular weights and high viscosity. These compounds also do not have good compatibility. Thus, they cannot be added in large amounts. The compounds which have a smectic phase below the nematic phase may be added in relatively larger amounts without incurring compatibility problems. Examples of such high-temperature liquid crystal compounds include compounds having the following formulae: ##STR4## wherein R and R' are linear alkyl or alkoxy groups and X is H or F.
Generally, to prepare a liquid crystal composition having the required characteristics, liquid crystal compounds of each of the three types described above are combined.
In order to lower the driving voltage of a liquid crystal display device, it is necessary to reduce the threshold voltage. However, for a TN type liquid crystal display device, the following relationship exists between the threshold voltage (V.sub.th), the thickness of the liquid crystal layer (d), the spray elasticity constant (K.sub.11), the twist elasticity constant (K.sub.22), the bend elasticity constant (K.sub.33), dielectric constant anisotropy (.DELTA..epsilon.) and the dielectric constant in a vacuum (.epsilon..sub.0): ##EQU2##
Thus, in order to lower V.sub.th, a liquid crystal compound having a large, positive dielectric constant anisotropy (.DELTA..epsilon.) and a small elasticity constant is required. Conventional liquid crystal compounds possessing these characteristics include: ##STR5## wherein R is a linear alkyl group, X is F or CN, and Y is H or F.
Generally, V.sub.th is reduced proportionately with an increase in temperature and this decrease is steep, particularly, near the N-I point. This is due to the dependence of the elasticity constant on temperature. The dependence of the elastic constant of the liquid crystal composition on temperature is lower for the liquid crystal compounds of the Nn type than for those of the Np type. Thus, the addition of an Nn type liquid crystal compound is effective in reducing the dependency of V.sub.th on temperature. The dependency of V.sub.th on temperature may also be reduced by adding a high-temperature liquid crystal compound. However, the addition of a high-temperature liquid crystal compound would increase the V.sub.th of the resulting composition.
The following relationship exists between steepness (.gamma.), threshold voltage (V.sub.th) and saturation voltage (V.sub.sat) ##EQU3## Generally, V.sub.th is the voltage at 10% luminance and V.sub.sat is the voltage at 90% luminance. Although the value .gamma.=1 is ideal, the value of .gamma. is generally between about 1.3 and 1.5 and it is difficult to develop a liquid crystal compound having a lower value of .gamma.. Steepness (.gamma.) is related to K.sub.33 /K.sub.11, and steepness decreases proportionally as the ratio K.sub.33 /K.sub.11 decreases for TN liquid crystal display devices. Pyrimidine compounds, in general, have small .gamma. values.
The dependence of the V-I.sub.0 characteristic of a TN type liquid crystal display device on the visual angle is due to the pretilt which occurs between the oriented plane and the liquid crystal material. Pretilt is a problem unique to TN type liquid crystal display devices and can be reduced by decreasing the thickness (d) of the liquid crystal layer or by decreasing the birefringence (.DELTA.n) of the liquid crystal composition.
For TN type liquid crystal display devices, transmittance (T), in the absence of an electric field, is expressed by the following formula: ##EQU4## where u is represented by the following formula: EQU u=2d.DELTA.n/.lambda.
In the above formula, T=0 when u.perspectiveto.2, 4, 6, . . . The dependency of the V-I.sub.0 characteristic on visual angle is smallest when u 2. Due to the difficulty of forming a liquid crystal composition having a small value of .DELTA.n, previously the only method of reducing the dependency of V-I.sub.0 on visual angle has been to decrease the value of d. However, current manufacturing techniques permit no appreciable decrease in the value of d. Thus, liquid crystal cells are manufactured with values of d and .DELTA.n so that u.perspectiveto.4. Accordingly, the development of liquid crystal compounds having small .DELTA.n values would result in the formation of a TN type liquid crystal cell having a dependency of V-I.sub.0 on visual angle low enough to satisfy the equation u.perspectiveto.2.
The initiating speed .tau..sub.ON and the trailing speed, .tau..sub.OFF, which are measures of the response speed of a twisted nematic liquid crystal display device may be calculated by the following formulae: EQU .tau..sub.ON =.eta.d.sup.2 /(.epsilon..sub.0 .DELTA..epsilon.V.sup.2 -.pi..sup.2 K) EQU .tau..sub.OFF =.eta.d.sup.2 /K.pi..sup.2
where
Thus, a high speed response is obtained by decreasing the thickness (d) of the liquid crystal layer, lowering the viscosity (.eta.) of the liquid crystal material and raising the value of the elasticity constant (K). From the perspective of the liquid crystal material, the response speed may be raised by using a material having as small a value of .eta./K as possible. Compounds having small .eta./K values are generally known as viscosity decreasing agents and include compounds having the following formulae: ##STR6## wherein R and R' are linear alkyl or alkoxy groups.
In a liquid crystal display device having a time sharing driving system, the relationship between the steepness (.gamma.) and the maximum allowable number of scanning lines (N) is expressed by the following formula: ##EQU5## From the above formula, it is evident that the value of N increases proportionately as the value of .gamma. approximates 1. Generally, when a time sharing driving system is used in a liquid crystal display device, the voltage margin (M) is used to evaluate the effectiveness of a given liquid crystal composition used as the liquid crystal material. The voltage margin (M) is defined by the following formula: ##EQU6## wherein V.sub.th and V.sub.sat are the voltage at 10% and 50% of light transmittance, respectively; T is the temperature; and .phi. is the visual angle (with the front side 0.degree.).
The above formula shows that three characteristics are important to obtain a large-scale time-sharing driving system:
(1) The steepness (.gamma.) should be sharp.
(2) The dependence of V.sub.th and V.sub.sat on temperature should be small.
(3) The dependence on visual angle (.alpha.) should be small.
Compounds having the following formulae are known to have the above desired characteristics: ##STR7## [H-M Vorbrodt, et. al., Mol. Cryst. Liq. Cryst., 123, 137 (1985)] ##STR8## [E. Kleinpeter, et. al., Tetrahedron, 44, 1809 1988)]
Although the above nematic liquid crystal compounds have high N-I points and small values of .DELTA.n and .gamma., they do not have good compatibility with conventional liquid crystal compositions.
Accordingly, it is desirable to provide a nematic liquid crystal material and compositions having large positive .DELTA..epsilon. and small .DELTA.n, which have good compatibility with other liquid crystal compounds, and which may be used in a large-scale time sharing driving system at low voltage.